B&B Electronics 422LP9N - Datasheet - Manual

Document No. 422LP9N0812 - pg. 1/2
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Port Powered RS-232 to RS-422 Converter
Model 422LP9N
The 422LP9N is a port-powered two-channel RS-232 to RS-422 converter. It converts TD and RD RS-232
lines to balanced RS-422 signals. The unit is powered from the RS-232 handshake lines DTR and RTS. One of
these handshake lines must be asserted (high) in order to power the unit (See Table 1). The RS-422 driver is
enabled when RTS is asserted. Therefore, RTS must be asserted in order to transmit data. The RS-422 receiver
is always enabled. If DTR is always asserted and the RTS is used to control the driver, the 422LP9N can be used
as an RS-232 to RS-485 4-wire converter. To insure reception, it is recommended that DTR is asserted.
In order to maximize the amount of power available to the RS-422 driver, the RS-232 handshake lines
are not looped back (tied together). As a result the following handshake lines will appear as disasserted (low):
CTS, DCD, and DSR. Care should be taken to in
sure that any software being used doesn’t require any of these
handshake lines be asserted. If existing software requires any of the handshake lines to be asserted, you can loop
back the required handshake lines in your cable.
Table 1
* NOTE: Low = disasserted and High = asserted
Connections
The 422LP9N can be connected in three different types of systems: RS-422 (Figure 1), four-wire RS-485
(Figure 1), and two-wire RS-485 (Figure 2). Regardless of the system, the 422LP9N must be connected with the
proper polarity. With no data is being sent and the driver enabled, the RS-232 line should be negative and the
TD(A) should be negative with respect to TD(B).
Proper operation of any RS-422 system requires the presence of a signal return path between the signal
grounds of the equipment at each end of an interconnection. This circuit reference may be established by a third
conductor connecting the common leads of devices, or it may be provided by connections in each equipment to
an earth reference. When the circuit reference is provided by a third conductor, the connection between the signal
grounds and the third conductor should contain some resistance (e.g. 100 ohms) to limit circulating currents when
other ground connections are provided for safety.
RTS
State
DTR
State
Functions Possible
(when port-powering unit)
Low Low none
Low High Receive
High Low Transmit, Receive
High High Transmit, Receive